VARIATION IN EPIDERMAL COLOR OK NOJADES. 28 1 



and from the current itself, especially when the latter carries sus- 

 pended matter. Darkening then might be due to the use of minute 

 particles of the silt in the shell building activities of the animal or 

 to their external deposition on the shell. This general proposition, 

 that the silt is indirectly or indirectly responsible for the blackening 

 of the shell, is borne out by the fact that in L. Erie where there is 

 relatively less silt, the shells are lighter and clearer ip. epidermal 

 colors. 



If we now endeavor to relate the facts stated to those seemingly 

 furnishing a reasonable hypothesis for the change in nacreous 

 color, we may tentatively suggest that shells are yellow or yellowish 

 green upstream and in the tributaries on account of the yellow 

 oxide of iron (Fe203) they may contain. Downstream the inclusion 

 of greater amounts and variety of organic matter in the silt darkens 

 them as organic matter darkens clay. That the degree of pigmenta- 

 tion seems to depend largely on the environment colors. There 

 appeared to be no regular sequence of development of epidermal 

 color beyond the fact that most shells are yellowish when young, 

 and with age become blackish or brownish. A shell from a given 

 locality may have old age colors when it is still comparatively 

 young, while another locality may show shells retaining juvenile 

 colors to an advanced maturity. We have also noted that shells 

 darken with age in the tributaries and headwaters, that is, quite 

 independent of their geographical locality. A fair reason is presented 

 when we combine the time element with the factors stated above, 

 and take into consideration the physical characters of the shells. 

 While the water in the tributaries is swifter, erosion of the shells 

 may be slowed down, for such characters as greater compression 

 there, (as has been confirmed by many investigators) enable it to 

 present less surface to the eroding waters, and while silt is present 

 it is riot in abundance and variety as is the case further down 

 stream. If silt plays any part in the coloration of the epidermis, 

 it is evident that a longer time will be required to affect the shell, 

 and it follows, that age for age, colors will be Ughter in the tribu- 

 taries than further down stream. A convenient analogy here is 

 that applying to the bark of certain trees. The cork will become 

 black more quickly in an industrial community under the influence 

 of smoke or chemicals in the air, than it will in the virgin forest, 

 but ultimately it becomes black in either locality. As the shells 

 were first thoroughly scrubbed before making color comparisons, 



